1. Field of the Invention
The present invention relates to a charged particle beam apparatus, more particularly to a charged particle beam apparatus which corrects an optical axis deviation of a charged particle optical system and is suitable for obtaining a high resolution image.
2. Discussion of the Background
A charged particle beam apparatus typified by a scanning electron microscope acquires desired information, for example, a sample image, from a sample by scanning a finely converged charged particle beam thereonto. Because, in such a charged particle beam apparatus, lens aberration occurs and a resolution of the sample image decreases with an optical axis deviation relative to a lens, a high precision axis alignment is required to acquire a high resolution sample image. Therefore, excitation current of an objective lens and the like were periodically changed in a conventional axis alignment, and operational conditions of a deflector (aligner) for the axis alignment was manually adjusted so as to minimize a motion thereof at this time. As a technology for performing the axis alignment automatically, there is one disclosed in Japanese Patent Laid-Open No. 2000-195453. According to descriptions of this gazette, disclosed is the technology for changing an excitation set value of an alignment coil based on a transition of an electron beam radiation position which changes between two excitation conditions of the objective lens. Moreover, in Japanese Patent Laid-Open No. 2000-331637, a technology is disclosed, which performs a focus correction based on a detection of a position deviation between two electron microscope image obtained under different optical conditions.
If the lens deviates from a center of an astigmatism corrector for performing an astigmatism correction of a charged particle beam, a field of view moves in aligning astigmatism, and the alignment of the astigmatism becomes difficult. Therefore, another aligner (deflector) which controls the position of the charged particle on the sample in association with an operation of the astigmatism corrector is provided, and a motion of an image relative to a change of a set value of the astigmatism corrector is cancelled, thus performing a correction of the field of view so that an observed image does not move in aligning the astigmatism. At this time, though a signal in proportion to the set value of the astigmatism corrector is input to the aligner for correcting the field of view, a proportional coefficient must be determined so as to cancel the motion of the image in aligning the astigmatism. To perform this alignment, an operation, in which the set value such as current and the like of the astigmatism corrector was periodically changed and a proportional coefficient to minimize a motion of an image at this time is determined, was carried out.